The co-inventor of the present invention previously has developed a process for the manufacture of high grade devulcanized rubbers as disclosed in U.S. Pat. No. 5,731,358. This earlier process has been produced on a commercial scale to make devulcanized rubber for use in the manufacture of automotive molded parts. The resulting products using this devulcanized rubber have improved physical characteristics over previous recycled rubber products. In addition, the manufacturing costs associated with producing these products have been reduced significantly.
During the course of designing specific formulations for certain types of seals using the devulcanized rubbers of the prior process, it was found that thermoplastic rubbers are more commonly used in the manufacture of seals than the conventional rubbers. In view of this finding, various compositions comprising a mixture of thermoplastics and recycled rubbers were developed, tried and tested using a similar processing technique to that which was used in the devulcanization process of U.S. Pat. No. 5,731,358. At the same time, improvements to the process were developed to modify the processing temperatures and the sequence of incorporating various reagents and additives to the main components. These improvements resulted in a unique phenomenon of changes in flow characteristics relating to the rubber viscosity. This phenomenon is the progressive change in viscosity which first drops and then increases during the course of blending the rubber composite with the plastic resin at elevated temperatures in the presence of a select group of chemicals and additives. This phenomenon is termed "dynamic de-vulcanization", which refers to the devulcanization of the vulcanized rubber scrap material while blending with a thermoplastic resin. Thus, the present invention can be considered an improvement and extension of the process disclosed in the previous U.S. Pat. No. 5,731,358, as well as further efforts by the present inventors to address the disposal problems of rubbers scraps, most of which are now sent to landfills with the prospects of serious environmental consequences to the soil and ground water.
U.S. Pat. No. 4,130,535 to Coran et al. discloses the preparation of "Thermoplastic Vulcanizates" (TPV) from ground powders of completely vulcanized rubber blended with polypropylene and polyethylene. In Coran et al., TPV compositions are based on virgin rubbers that first are vulcanized, and virgin olefinic resins. In contrast, the present invention uses a devulcanized rubber component based on post industrial rejects and post consumer scraps, the devulcanization of the rubber being accomplished in a single stage. A similar thermoplastic rubber composition and the method of its manufacture is described in U.S. Pat. No. 3,758,643 to W. K. Fisher. Fisher's method and material differs from the present invention in that the Fisher process also uses virgin rubber as a starting material which is "dynamically" vulcanized first and then blended with a plastic resin.
U.S. Pat. Nos. 5,191,005 to and 5,856,399 to (both assigned to Misui Corporation) teach processes and compositions for preparing a thermoplastic elastomer based on a blend of olefinic plastics and two polymers, one of which is cross-linkable by peroxide and the other of which is not. The Misui patents differ from the present invention in that they also use virgin rubbers as the starting material, rather than used and recycled rubber scraps. In addition, neither patent discloses the "dynamic de-vulcanization" technique of the present invention. In U.S. Pat. No. 5,597,867 to Tsujimoto et al., a composition comprising a partially cross-linkable rubber portion of a rubber/plastic blend with an organosiloxane. As with the other patents, Tsujimoto et al. also uses virgin rubber and does not use a dynamic de-vulcanization technique. U.S. Pat. No. 5,934,558 to Kim et al. discloses compositions comprising styrenic polymer, chlorinated ethylene, a modified ethylene-propylenic rubber and a grafted vinyl copolymer. Kim et al. does not use a recycled rubber and requires the use of additional starting materials in the blend.
U.S. Pat. No. 5.157,082 to Lane D. Johnson describes a process using vulcanized scraps, including used tires, which first are finely ground and then blended with thermoplastic resins to produce extremely hard composites with Hardness indices ranging from Shore D levels of 35 to 45. Johnson also describes a process for adding SEBS copolymers to reduce the hardness level to about Shore A 60. The Johnson process differs from the present invention in that (a) the vulcanized scraps have to be finely ground, and (b) the subsequent mixing process is a straight blending without "devulcanization" of the scrap vulcanized rubber which is evident from lower physical properties, in particular the tensile strength of Johnson's samples from the embodiments. The dynamic devulcanization in the present invention contributes to greatly enhanced tensile strength at any given hardness level, and other desirable attributes described in later sections of this application. Two prior patents teach the use of recycled rubbers in the preparation of thermoplastic rubber compositions. Specifically, U.S. Pat. No. 5,733,943 to Doan discloses a process and composition for the manufacture of a street sign material using reground rubber from used tires in a blend with a styrene-butadiene copolymer and polypropylene or polyethylene mixed in a heated extruder to produce the final product. In U.S. Pat. No. 6,015,861 to Mertzel et al., a composition comprising finely ground cross-linked rubber with a compatibilizer and thermoplastic material is prepared by a simple mixing technique using an extruder or other form of mixer. Both Doan and Mertzel et al. differ from the present invention in that neither subjects the used ground rubber component to devulcanization. In the present invention, it has been found that the use of a devulcanized used rubber greatly increases the interfacial adhesion between the rubber component and the thermoplastic component, thereby resulting in improved overall physical properties, and in particular the tensile strength, of the final product.
None of the prior art processes and compositions discussed above disclose or teach a process using "dynamically de-vulcanized" vulcanized rubber derived from recycled rubbers which have been prepared through a milling technique, rather than being ground into a fine powder without devulcanization. Thus, a need still exists for a process for the manufacture of thermoplastic rubbers from vulcanized rubber scrap materials and olefinic plastic resins in the presence of at least one devulcanizing additive. In addition, such a process should utilize particular devulcanizing additives which promote interfacial adhesion between the thermoplastic component and the rubber component. Moreover, such a process should improve the physical properties and surface finish of the resulting products relative to products obtained from the prior art methods.